Conventionally, a ceramic metal circuit substrate having an insulation and electrode function has been used in a field of mounting power electronics. The ceramic metal circuit substrate is used as a substrate of a semiconductor module having the insulation and electrode function, for instance, in a semiconductor module structure including a semiconductor module and a radiating member such as a radiation fin. In addition, the semiconductor module structure has been required to have high radiation properties along with the tendency for the power of a semiconductor module to increase in recent years.
In such insulation and electrode fields, a substrate which contains alumina (Al2O3) or aluminum nitride (AlN) as a main component has been conventionally used as a ceramic substrate.
However, an alumina substrate has the low thermal conductivity of about 18 W/m·K, and accordingly is insufficient in radiation properties. On the other hand, an AlN substrate has the high thermal conductivity of about 200 W/m·K but has low strength, and accordingly is insufficient in heat-cycle resistance characteristics.
Here, the term heat-cycle resistance characteristics means the resistance to the occurrence of a crack which is formed along a peripheral shape of a metal circuit out of ceramic substrate portions of a sample, when the sample of the ceramic metal circuit substrate is produced by forming the metal circuit on a surface of the ceramic substrate, and the sample is subjected to a predetermined heat cycle test.
Thus, it is difficult for a conventional alumina substrate or AlN substrate to satisfy the characteristics of both thermal conductivity and strength. For this reason, a high thermal-conductivity silicon-nitride substrate has been developed as a ceramic material which satisfies the characteristics of both thermal conductivity and strength.
For instance, Japanese Patent Laid-Open No. 2009-120483 (Patent Document 1) describes a high thermal-conductivity metal circuit substrate made from silicon nitride, in which a leakage current is reduced by controlling a diameter of a pore in a grain boundary phase.
The metal circuit substrate made from silicon nitride described in Patent Document 1 is a substrate which has been produced by bonding a copper circuit board to a high thermal-conductivity silicon-nitride substrate through a brazing filler material of an Ag—Cu—Ti-based active metal. Generally, the high thermal-conductivity silicon-nitride substrate contains silicon nitride as its main component, and accordingly a three-point bending strength is as high as 200 MPa or higher. Because of this, the metal circuit substrate made from silicon nitride, which has been produced by bonding the high thermal-conductivity silicon-nitride substrate to the metal circuit board such as a copper plate, has adequate heat-cycle resistance characteristics.
In addition, Japanese Patent Laid-Open No. 2003-192462 (Patent Document 2) discloses that the obtained high thermal-conductivity metal circuit substrate made from silicon nitride can endure a heat-cycle resistance test (TCT test) of 3000 cycles.
However, in order to manufacture the high thermal-conductivity metal circuit substrate made from silicon nitride described in Patent Documents 1 and 2, a bonding process of bonding the high thermal-conductivity silicon-nitride substrate to the metal circuit board is necessary, which needs a high manufacturing cost. Because of this, the high thermal-conductivity metal circuit substrate made from silicon nitride needs a high manufacturing cost.
Then, for the purpose of securing insulation properties of the semiconductor module structure while reducing the manufacturing cost, it has been investigated to use a method in which the high thermal-conductivity silicon-nitride substrate is not bonded to the metal circuit board, in other words, a method which does not produce the high thermal-conductivity metal circuit substrate made from silicon nitride.
For instance, it is proposed to use a high thermal-conductivity silicon-nitride substrate as a spacer for a semiconductor module structure having a pressure contact structure in Japanese Patent Laid-Open No. 2003-197836 (Patent Document 3). It is confirmed that the high thermal-conductivity silicon-nitride substrate according to Patent Document 3 can sufficiently endure as the spacer for the pressure contact structure such as a screw clamp or the like, because the strength and fracture toughness are high.
For information, a silicon-nitride sintered compact which contains a β-silicon-nitride (Si3N4) crystal as a main phase is generally used as the high thermal-conductivity silicon-nitride substrate. In addition, as the β-Si3N4 crystal, a crystal grain having a vertically long shape of which the aspect ratio is two or more is usually used. Here, the aspect ratio is a value obtained by dividing a length in a major axis direction of the crystal grain by a length in a minor axis direction of the crystal grain.
The high thermal-conductivity silicon-nitride substrate which contains β-Si3N4 as a main phase has a microscopic structure in which the β-Si3N4 grains that have the vertically long shape and have an average grain size, for instance, of about 2 to 10 μm are complicatedly entangled in such a state that the grains face in random directions. The high thermal-conductivity silicon-nitride substrate which contains β-Si3N4 as the main phase has such a structure, and thereby has high strength and fracture toughness.
When the surface of the high thermal-conductivity silicon-nitride substrate which contains β-Si3N4 as the main phase is microscopically observed, there exists unevenness on the surface of the silicon-nitride substrate. The unevenness is formed of protrusions that are parts of tips of a large number of complicatedly entangled β-Si3N4 grains, or the like. The parts of tips forming the protrusions have protruded from the surface of the substrate.
However, it is difficult to eliminate the unevenness even by mirror-polishing the surface to a surface roughness Ra of 0.05 μm or less. In addition, the mirror polishing also becomes a factor in an increase in manufacturing costs of the high thermal-conductivity silicon-nitride substrate, which is not preferable.
Thus, the high thermal-conductivity silicon-nitride substrate having the microscopic unevenness on the surface has such a problem that a crack tends to be easily formed in the silicon-nitride substrate. The crack initiates from the salient when the high thermal-conductivity silicon-nitride substrate is used in such a state that a pressure contact stress is applied onto the microscopic unevenness, particularly onto the salient for a long period of time.
In addition, the high thermal-conductivity silicon-nitride substrate having the microscopic unevenness on the surface produces a microscopic gap between the surface of the high thermal-conductivity silicon-nitride substrate and another member, when a module structure such as a semiconductor module structure having the pressure contact structure is formed by bringing the surface of the high thermal-conductivity silicon-nitride substrate into contact with the another member such as a pressing member. However, the pressing member is generally constituted by a hard material such as a metal, and accordingly, it is difficult to fill the microscopic gap with the material of the member by deforming the shape of the surface of the pressing member side so as to match the unevenness of the surface of the high thermal-conductivity silicon-nitride substrate. For this reason, if the gap is formed between the surface of the high thermal-conductivity silicon-nitride substrate and the pressing member in the module structure having the pressure contact structure, the gap becomes an obstructive factor in thermal conduction and lowers the radiation characteristics of the module structure.